The Uniform Solar, Hydronics and Geothermal Code (USHGC) includes provisions for alternative energy sources for transferring energy in a geothermal system for space heating and cooling. 

As we elaborated in Part 1 of this series, the sun’s energy can power equipment using solar photovoltaic (PV) systems or heat water or other fluids using solar thermal systems. These are examples of how the sun’s energy can be used to power equipment that can be used to move the heat transfer medium to and from a hydronic system. 

As stipulated in Parts 1 and 2, Figure 1 illustrates how the solar PV system can power the various components in a hydronic system. Solar PV is used to power the equipment, and solar collectors are used for heating loads like hydronic heating.

This article focuses on the geothermal system, as covered in Chapter 7 of the USHGC, which applies to geothermal energy systems that are part of heating, cooling, ventilation, refrigeration and air-conditioning systems. 

The regulations of Chapter 7 apply to geothermal energy systems such as, but are not limited to, building systems coupled with a ground-heat exchanger, submerged heat exchangers using water-based fluid as a heat transfer medium or groundwater (well). This chapter’s regulations govern the construction, location and installation of geothermal energy systems.

With the addition of geothermal energy systems, Figure 2 shows the solar collector as an energy source, and the geothermal energy system is also being used as another source for the heating load, such as a hydronic system. The solar PV shown in Figure 1 is used to power the equipment. This is how the USHGC puts the pieces together as one easy-to-use, stand-alone document for solar, hydronics and geothermal.

The U.S. Environmental Protection Agency estimates that more than a million homes use ground-source heat pumps for their heating and cooling needs. Heat pumps can effectively deliver energy for both heating and cooling loads. Heat pumps are generally constrained by the area available to install underground piping loops. For larger-scale applications such as large buildings or district heating, geothermal hot water or geothermal steam can be a particularly efficient source of renewable heat if it is available.

The USHGC geothermal chapter consists of four parts: general requirements, and closed-loop, open-loop and direct exchange (DX) systems. 

General Requirements

Part I of Chapter 7 applies to geothermal energy systems such as, but not limited to, building systems coupled with a ground-heat exchanger, submerged heat exchangers using water-based fluid as a heat transfer medium or groundwater (well). The regulations of Chapter 7 govern the construction, location and installation of geothermal energy systems. 

It is crucial that the engineer or licensed professional has the appropriate certifications to ensure the system’s design performs as intended but that all safety measures have been followed. The industry offers many credentials; some jurisdictions have additional professional qualifications that must be met.

Proper installation practices are very important for a properly functioning geothermal energy system, and Section 707.0 of the USHGC provides the minimum necessary provisions. For example, documents for permits and construction documents shall be submitted to the authority having jurisdiction prior to the construction of a building system or water well. 

The mechanical equipment, accessories, components and materials used shall be recommended by the manufacturer for the specific use. The site survey shall identify the physical limitations of the land area, including its extent, structures, existing wells of all types, the proximity of other existing ground-source heat pump systems, pavements, trees, grading, ponds, waterways, easements, overhead and underground services, septic systems, any identified septic repair areas, utility rights of way, and any other elements that may affect an open-loop configuration. 

If necessary, permission shall be obtained from any adjoining property owner(s), as evidenced by the registration and approval of a formal easement meeting requirements of the authority having jurisdiction. It shall be received prior to installing any open-loop system extending into, cross or interfere with the equipment or rights of way of utilities, jurisdictions and other property owners. 

The site survey shall include a subsurface investigation meeting the requirements for an open-loop heat exchanger. A subsurface investigation shall be performed in accordance with the USHGC, as determined by the registered design professional or certified person conducting the site survey.

The water well logs and other geological records shall be used to anticipate the subsurface conditions of the aquifer and its potential supply of fresh water, multiple aquifers, saltwater intrusions, contaminated soils and groundwater, hazardous gases and any interference with neighboring water wells and ground-source heat exchangers. Geological issues such as permafrost conditions and building stability shall be considered when reviewing available records.

These general requirements are just the tip of the iceberg; trenching, sleeve, ground-heat exchangers, piping installations, etc., also are required. Refer to Section 707.0 for the entire general requirements for the installation of a geothermal energy system.

Closed-Loop Systems

The USHGC defines a closed-loop geothermal system as follows:

“Geothermal Energy System, Closed-Loop. A continuous, sealed, underground or submerged heat exchanger through which a heat-transfer fluid passes.”

Part II of the USHGC has the minimum provisions for closed-loop systems. 

“710.0 General.

“710.1 Applicability. Part II of this chapter shall apply to geothermal energy systems such as, but not limited to, building systems coupled with a closed-loop system using water-based fluid as a heat transfer medium.”

A closed-loop geothermal system continuously circulates a heat transfer solution through a buried or submerged piping system that connects to a heat pump to provide the heating or cooling load. The loop is filled once and requires only a moderate amount of solution. 

Below are some minimum provisions addressed in the USHGC for closed-loop systems. The references to Section 710.3 and Table 710.3 send the reader to the appropriate fitting requirements applicable to any geothermal system. 

“710.3 Borehole Piping and Tubing. Borehole piping or tubing for vertical and horizontally drilled closed-loop systems shall have a minimum wall thickness equal to SDR-11 and shall have a minimum pressure rating of not less than 160 psi (1103 kPa) at 73 F (23 C).

“710.4 Underground Fittings. Underground fittings for closed-loop systems shall be in accordance with Section 703.3 and Table 703.3.

“710.5 Verification. For closed-loop systems, the system shall be flushed of debris and purged of air after completion of the entire ground-heat exchanger. Flow rates and pressure drops shall be compared to calculated values to assure no blockage or kinking of the pipe. A report shall be submitted to the owner to confirm that the loop flow is in accordance with the construction documents.

“710.6 Vertical Bores. Vertical bores shall be drilled to a depth to provide complete insertion of the u-bend pipe to its specified depth. The borehole diameter shall be sized for the installation and placement of the heat exchange u-bend and the tremie used to place the grouting material. CSA/IGSHPA C448 shall be used for vertical loop depth and borehole diameter sizing guidance. The u-bend joint and pipe shall be visually inspected for integrity in accordance with the manufacturer’s installation instructions. The u-bend joint and pipe shall be pressurized to not less than 100 psi (689 kPa), not to exceed the pressure rating of the pipe at the test temperature, for [one] hour to check for leaks before insertion into the borehole.”

Open-Loop Systems

Part III of Chapter 7 applies to open-loop geothermal energy systems such as, but not limited to, building systems coupled with groundwater (well) or surface water open-loop using water-based fluid as a heat transfer medium. The regulations of Chapter 7 govern the construction, location and installation of geothermal energy systems. 

An open-loop geothermal system circulates groundwater directly from a nearby aquifer to an indoor geothermal heat pump. After the water leaves the home, it’s circulated back through a discharge well and into a local groundwater system or approved location as permitted by the authorities having jurisdiction. 

The USHGC defines an open-loop geothermal system as follows:

“Geothermal Energy System, Open-Loop. A liquid-source system that uses groundwater or surface water to extract or reject heat.” 

Section 702.0 has the minimum provisions for a groundwater system and Part III of the USHGC has the minimum requirements for an open-loop geothermal system. Unlike an open-loop system, the fluid is not contained within the piping system.

“702.0 Groundwater Systems.

“702.1 General. The potable water supply connected to a groundwater system shall be protected with an approved backflow prevention device. The connection of a discharge line to the sanitary or storm sewer system, or private sewage disposal system, shall be in accordance with the plumbing code or in accordance with the Authority Having Jurisdiction.”

When using a groundwater system, it is crucial that the potable water is protected from contamination with a backflow prevention device. Plumbing codes such as the Uniform Plumbing Code, which is also accredited by the American National Standards Institute (ANSI), have the appropriate backflow requirements, such as a double-check valve. 

Direct-use geothermal systems use groundwater that is heated by natural geological processes below the earth’s surface (see Figure 3). This water can be as hot as 200 F or more. Bodies of hot groundwater can be found in many areas with volcanic or tectonic activity. In locations such as Yellowstone National Park and Iceland, these groundwater reservoirs can reach the surface, creating geysers and hot springs. 

One can pump hot water from the surface or from underground for a wide range of useful applications. Deep geothermal sources provide efficient, clean heat for industrial processes and some large-scale commercial and agricultural uses. 

Direct Exchange

Part IV of Chapter 7 applies to direct exchange geothermal energy systems such as, but not limited to, building systems coupled with a direct-exchange (DX) closed-loop using refrigerant as a heat transfer medium. The regulations of Chapter 7 govern the construction, location and installation of geothermal energy systems.

The USHGC defines a direct-exchange system as follows:

“Direct Exchange (DX). A ground-source heat pump that circulates a refrigerant through a closed-loop system.”

During heating mode, the heat is transferred from the earth to the ground loop filled with refrigerant. The heated refrigerant is then returned to the home through a closed-loop system. The heat is transferred from the refrigerant to the heating system. 

This process is reversed during the cooling mode to remove heat from your home: Heat is transferred from your home, cooling the space, to the earth via the buried ground-loop system. This heat exchange process is efficient, effective and reliable.

“715.3 DX Systems. Copper pipe and tubing installed for DX systems shall be manufactured in accordance with ASTM B280 and copper fittings in accordance with ASME B16.22. Joints shall be purged with an inert gas and brazed with a brazing alloy having 15 percent silver content in accordance with AWS A5.8. Underground piping and tubing shall have a cathodic protection system installed.

“715.5 Indoor Piping. For DX systems, joints shall be purged with an inert gas and brazed with a brazing alloy having 15 percent silver content in accordance with AWS A5.8.

“715.6 On-Site Storage. For DX systems, copper piping and fittings shall be stored to prevent physical damage, contamination, and each pipe or tubing shall be pressurized with an inert gas and sealed with a cap.

“715.7 System Start-Up. DX system start-up shall be in accordance with Section 708.0 and the following:

“(1) DX systems shall be pressurized using nitrogen for not less than [one] hour. There shall be no allowable variance to the test pressure after being corrected for ambient temperature changes during the test. The test pressure shall not exceed 150 psig (1,034 kPa) when pressure-testing the compressor unit and indoor system components.

“(2) DX systems shall have permanent-type labels installed and affixed on the compressor unit with the refrigerant type and quantity.

“(3) For DX systems, refrigerant liquid and vapor lines from the loop system shall be identified and tagged.

“715.8 DX Piping. DX piping shall be installed in accordance with approved plans and specifications, including provisions for cathodic protection.”

2024 Edition

The USHGC addresses the green energy technologies available for a more efficient heating and cooling source. All three components of this green energy technology are being improved and updated. The 2024 edition is being developed and will be released in early 2024.

Proposed changes to the code are being reviewed and approved by a balanced consensus body that reviews all proposals and comments via the ANSI-accredited code development process. 

The geothermal provisions addressed in the USHGC — developed by an experienced technical committee in the solar, hydronics and geothermal field — are the most advanced and updated provisions in the geothermal industry. 

The code includes the critical installation requirements to help make solar, hydronic and geothermal energy systems safe and efficient. Requirements are written using enforceable language so the inspector does not necessarily need to have decades of experience to ensure adherence to these minimums.

The USHGC is not a design guide; designers, contractors and installers should still seek competent professional training related to their scope of work in solar, hydronics or geothermal energy systems. However, code requirements can help all parties involved avoid serious and costly mistakes. l

Hugo Aguilar, PE, is the vice president of codes and standards at IAPMO. His responsibilities include overseeing the IAPMO codes and standards development department.

Cary Smith, CGD, CEM, CEA, CGI and AEE Fellow, is a managing member and U.S. operations director of The GreyEdge Group and president/CEO of Sound Geothermal Corp. He currently serves on the IAPMO 2024 USHGC Technical Committee, and the IAPMO 2024 Uniform Mechanical Code Technical Committee. Smith has served on the IAPMO Solar Standards Committee and Hydronics Standard Committee. He also serves on the Utah Energy Office Beneficial Electrification Committee.